If your vehicle is showing engine cranks but won't start, here are the most likely causes and the parts you'll need to fix each one.
The engine is not receiving fuel from the gas tank. You will typically notice the absence of the low humming sound the fuel pump makes when you first turn the key to the 'ON' position.
This sensor tells the engine computer the exact position of the engine internals. If it fails, the computer will not know when to fire the spark plugs or fuel injectors, resulting in a continuous crank with no start.
The vehicle's security system does not recognize the microchip in your key. To prevent theft, the computer intentionally disables the fuel injectors or ignition system.
The engine is getting fuel but lacks the spark needed to ignite it. This can be caused by a failed main ignition module, blown ignition fuses, or severely fouled spark plugs.
The belt or chain that synchronizes the top and bottom halves of the engine has snapped. The engine will usually sound completely different when cranking, spinning much faster than normal due to a total loss of compression.